This invention belongs to the field of catalytic reforming processes.
Catalytic reforming of naphtha feed stocks has long been known in the petroleum industry. Most naphtha feeds contain large amounts of naphthenes and paraffins and consequently have low octane numbers. By means of various hydrocarbon conversion reactions, catalytic reforming has improved the octane number of naphtha feed stocks. Some of the more important conversion reactions that take place during catalytic reforming are dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to naphthenes and aromatics and isomerization of normal paraffins to isoparaffins. A less desirable reaction which also occurs during reforming is the hydrocracking of paraffins, naphthenes and dealkylation of alkylaromatics to gaseous hydrocarbons such as methane and ethane.
The above reforming reactions have previously been catalyzed by catalysts comprising porous supports, such as alumina, that have dehydrogenation promoting metal components impregnated or admixed with the support. Platinum on alumina and, more recently, multimetallics, including bimetallics, such as platinum and rhenium on alumina, are examples of these catalysts. Representative multimetallic reforming catalysts are described in U.S. Pat. Nos. 2,848,377, 3,415,737 and 3,953,368, among others.
Other known reforming catalysts are based on zeolites containing a noble metal component such as platinum. U.S. Pat. No. 4,582,815 describes a method for preparing a silica-bound zeolite catalyst composition of improved crush strength relative to other silica-rich extrudates. According to this method, a mixture of silica and a zeolite such as ZSM-4 (Omega), ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, Beta, X, Y, L, ferrierite, mordenite, dachiardite, clinoptilolite, offretite, erionite, gmelinite, chabazite, etc., is mixed with water and an alkali metal base such as sodium hydroxide or a basic salt such as an alkali-metal carbonate, borate, phosphate, silicate, etc., as an extrusion aid followed by mulling, extruding and subsequently drying the extrudate. It is thought that substitution of alkali metal for hydrogen in the silanol groups on the surfaces of siliceous materials such as the foregoing zeolites is responsible for their improved crush strength. The resulting extrudate is said to possess superior crush strength and sufficient integrity to withstand treatment with acids so that it is possible to steam, acid extract or calcine them. To avoid trapping the alkali metal of the extrusion aid in the extrudate, the alkali metal is ordinarily removed by exchange under acidic conditions using dilute nitric acid in 1M ammonium nitrate solution. The silica-bound zeolite catalyst of U.S. Pat. No. 4,582,815 is indicated to be useful in a variety of hydrocarbon conversions including reforming.
In the method for preparing a low acidity refractory oxide-bound zeolite catalyst composition described in commonly assigned, copending U.S. patent application Ser. No. 44,639, filed May 1, 1987, a homogeneous mixture of a zeolite, e.g., a large pore zeolite such as ultrastable Y (USY) zeolite, water and a low acidity refractory oxide binder, e.g., silica, which contains at least an extrusion-facilitating amount of the binder in a colloidal state and which is substantially free of added alkali metal base and/or basic salt, is formed into an extrudable mass, the mass is extruded and the resulting extrudate is dried and calcined. The silica-zeolite composite obtained in this way is disclosed to be useful in catalytic reforming operations, among others.